This invention relates to a damper device for a motor and, more particularly, to a damper device adapted for use with a stepping motor.
When the stepping motor used in a control apparatus is rotationally stopped, it comes to a full stop after damping oscillations about a desired stop position. Therefore, to quickly stop the motor at a given position, a means for absorbing such oscillatory energy is required. One type of absorbing means is a damper device attached to a rotary shaft of the motor.
A conventional damper device is configured as shown in FIG. 8, that is, a magnet 3 made of a ring-shaped permanent magnet is provided on the outer periphery of a boss section 2 secured to a rotary shaft 1 of a motor, and the magnet 3 is engaged with a mass inertia section 4 (hereinafter referred to as "a mass section") extends into a concave portion 5 formed in the inner peripheral wall of the mass section 4 via a magnetic fluid 6.
The foregoing damper device utilizing the magnetic fluid needs no seal means for preventing leakage of the magnetic fluid and thus has the advantage that the damping effect is not degraded because the frictional resistance of a seal is not present.
Another conventional damper device is configured as shown in FIG. 9, in which the mass section 4 is mechanically supported rotatably on to the boss section 2.
That is, in this damper device, a rotary wheel 7 is secured to the end of a boss section 2 which is in turn secured to a rotary shaft 1 of a motor, the rotary wheel 7 is engaged with a concave portion 5 of a mass section 4 via a viscous fluid 6, and the mass section 4 is supported rotatably by the boss section 2. In this mechanically-supported type damper device, a V-ring-like seal means 7a for preventing leakage of the viscous fluid 6 is provided on the boss section 2.
This damper device of the mechnically-supported type has the advantage that the gap formed between the outer diameter of the rotary wheel 7 and the inner wall of the concave portion 5 of the mass section 4 can be made fixed, and that the structure is simplified.
However, among the foregoing conventional damper devices; the first magnetic fluid type has the disadvantage that the gap cannot be made fixed unless the magnetic fluid is high in viscosity because the magnetic fluid acting as the viscous fluid serves also as the bearing means of the mass section, and there is the drawback that the damping effect cannot be uniformly exerted because the viscosity of the fluid tends to decrease due to external heating, exspecially if the fluid possesses a high viscosity.
Further, if the load inertia or the rotor inertia of the motor varies, it is necessary to vary the inertia of the mass section or the viscosity of the magnetic fluid correspondingly.
That is, since the same gap not only functions as the bearing of the mass section but also causes the generation of the damping action, a variation in the weight of the mass inertia section results in an offset in the gap dimension (a narrow spacing and a wide spacing); thus, there is the drawback that the damping effect is not exerted uniformly. If a magnetic fluid of high viscosity is employed to make the gap uniform, the problem occurs that the damping effect is degraded by the influence of a variation in external temperature, as described above.
Further, since the viscosity of the magnetic fluid has some limits and therefore the gap being filled with the magnetic fluid must be made narrow, the accuracy of machining must be enhanced, thus increasing the cost.
On the other hand, the second damper device of the type in which the mass section is mechanically supported includes the seal means in the shaft section; thus, there is the disadvantage that due to the frictional resistance of the seal means, residual oscillations appear to degrade the damping effect, and there is the drawback that the actual stop position of the motor deviates from a target point due to the frictional resistance of the seal means.